3,6-Dichloropicolinic acid is a commercially useful herbicide (Johnston, U.S. Pat. No. 3,317,549) and 3,6-dichloro-2-(trichloromethyl)pyridine is known as a nitrification inhibitor (Goring, U.S. Pat. No. 3,135,594) and as a herbicide (Brett and Hodnett, U.S. Pat. No. 2,679,453). These compounds are not readily obtained by the pyridine ring chlorination of a 2-substituted pyridine because such chlorinations are not sufficiently selective to produce a preponderance of the desired 3,6-dichloro-2-substituted pyridine isomer in the mixture obtained. Alternative methods for introducing chlorine into the pyridine nucleus selectively at the 3 and 6 positions of 2-substituted pyridines depend upon the presence of amino or hydroxyl groups at those positions in the starting materials and the requisite materials are not commercially available.
A different approach to obtaining 3,6-dichloro-2-substituted pyridines which are convertible into 3,6-dichloropicolinic acid or 3,6-dichloro-2-(trichloromethyl)pyridine by simple known processes is to first prepare, from acyelic starting materials, a 2-hydrocarbyl-3,6-dichloropyridine. 2-Hydrocarbyl-3,6-dichloropyridines can be converted to 3,6-dichloropicolinic acid by oxidation using the general methods summarized in Abramovitch Pyridine and its Derivatives Supplement Part Two, pp 308-311 and Supplement Fart Three, pp. 268-272. 3,6-Dichloro-2-methylpyridine can also be chlorinated to obtain 3,6-dichloro-2-(trichloromethyl)pyridine using the methods described by Johnston and Tomita (U.S. Pat. No. 3,244,722). 3,6-Dichloro-2-(trichloromethyl)pyridine can be converted to 3,6-dichloropiecolinic acid by acid hydrolysis (U.S. Pat. No. 3,317,549).